Going to post my own answer to this even though I already accepted another answer, in order to maybe help others.
I discovered that, as pointed out by user6759519, the default font and font size used by WinForms applications can be different depending on the country or region Windows was installed.
For example, as I found out, the default font used by WinForms applications running on UK/US Windows is 'Microsoft Sans Serif 9pt', whereas on Korean Windows it is 'Gulim 9pt'.
This resulted in numerous panels having incorrect dimensions resulting in clipping, and various places where text did not fit in controls properly.
The solution was to programmatically override this by setting the default font back to Sans Serif in the constructor of the top-level panel in our application, just after the call to InitializeComponent, for example:
Font = new Font(
familyName: "Microsoft Sans Serif",
This setting automatically cascades down to all child panels and controls as far as I an tell, and pretty much everything now looks correct (I've also confirmed this in the Japanese edition of Windows 10).
Note: There may still be issues on some child panels even after overriding the default font, depending on the setting of some or all of the following in the child panel designer code:
Note: One more thing (after re-reading my question), I was never able to reproduce our Korean customer's issue by making my UK installation of Windows 'more Korean' (i.e. no setting I found changes the default font picked by WinForms) - instead I had to download Korean Windows from MSDN, and install it in VmWare. In order to use it without learning Korean(!), I was then able to change the default language into English, by navigating the relevant settings screens side by side with another PC running UK Windows - tricky but it is possible! I was then able to install Visual Studio and solve my issue programmatically as explained above).
Hope this helps.